Hi Kerosine
Yes, I believe you may be right. I asked a friend who has a copy of the AMM for his insight last night and whilst it doesn't explicitly describe the geometry of the mixer unit, its schematics appear to imply the presence of some sort of permeable partition within. It would seem this partition biases the output from pack 1 to the cockpit zone and that of pack 2 to the cabin zones. The description of the functionality of the mixer unit flap would seem to support this. Take a look for yourself:
http://i.imgur.com/D5gYeva.png